Ultra-fast X-ray tomographic measurement of activated sludge aerated by membrane and monolithic spargers

The removal of nitrogen and organic components by micro-organisms is an essential step in the biodegradable wastewater treatment plants. These biological reactions take place in large aerated cross flow reactors. In fact, up to 80 % of the total energy budget of wastewater treatment plants is consumed by the aeration process. Engineering and design of the plants are mostly based on empirical knowledge and limited instrumentation for single points of measurement. Thus, the complex hydrodynamic and biochemical processes in the large-scale vessels are not well understood, and there is a high potential for optimization of these processes. Recently, numerical tools, such as CFD, have been used for the design and prediction of improved operation of the reactors. However, the models used in the software show a significant deviation from experimental data and need improvement due to the complexity of the multiphase flows of aerated sludges (bubbles, microbial flakes, water). A detailed study has been carried out in a vertical column of 3.5 m in height at HZDR to obtain an improved understanding of the hydrodynamics of aerated sludges. To capture the temporal evolution of the rising bubbles in the opaque liquors the ultra-fast electron beam X-ray tomography system of HZDR was used. The target parameters are bubble size distribution, equivalent sauter diameter of the bubbles, bubble rise velocity and local gas hold-up under the variation of sparger type (rubber, monolithic material), gas flow rate, rheology of the fluid (deionized water, salty water, sludge) and height in the liquid column. Therefore enhanced image analysis algorithms, developed at HZDR, were applied to the reconstructed tomographic images, which are also presented in the paper. In the future, the experimental data set will be used as reference data for the improvement of numerical models of CFD software.